This invention relates, in general, to a semiconductor device and its method of manufacture, and more particularly, to a semiconductor device having improved electrical performance under high frequency operation.
The phenomenon of hot electrons trapped in gate oxides of silicon based metal-oxide-semiconductor field effect transistor (MOSFETs) is well documented and is widely accepted. The hot electron or charge trapping of MOSFETs alters certain MOSFET electrical characteristics such as, for example, threshold voltage.
A similar phenomenon has been observed during the high frequency operation of metal-semiconductor field effect transistors (MESFETs). However, because MESFETs do not have a gate oxide like MOSFETs, charge trapping in a MESFET occurs in a dielectric layer adjacent to the gate instead of under the gate. Under high frequency or radio frequency (rf) drive, the output power of a MESFET is expected to increase along with its output current. However, what is often measured for MESFETs operated at high rf drive is a decrease in output current and a decrease in output power, commonly known as "power slump" or "power sag". The degradation in MESFET performance can be attributed to the charge trapping phenomenon.
Accordingly, a need exists for a semiconductor device having improved electrical performance under high frequency operation which overcomes the disadvantages of the prior art. The semiconductor device should have reduced charge trapping and power slump during high frequency operation. The semiconductor device and its method of fabrication should be cost effective and manufacturable, should be easily integrated into an existing process flow, and should not significantly increase the cycle time of the process flow.